1) Field of the Invention
The present invention relates to a purification process of polyhydroxycarboxylic acid which is useful as a biodegradable polymer that can be substituted for conventional medical materials and general purpose resins.
2) Description of Related Art
Polyhydroxycarboxylic acid is excellent in mechanical, physical and chemical properties and additionally has a biodegradable function which can be degraded in the natural environment without giving a harmful effect on living organisms and is finally decomposed by microorganisms into carbon dioxide and water. Thus, polyhydroxycarboxylic acid plastics have recently received attention in various fields such as alternatives for medical materials and general purpose resins. Particularly, environmental issues are now closed-up and thus such plastics can conform to the needs of the era.
Generally, in the case of preparing polyhydroxycarboxylic acid by using hydroxycarboxylic acid such as lactic acid and glycolic acid as a raw material, polyhydroxycarboxylic acid can be obtained by conducting dehydration dimerization of hydroxycarboxylic acid and successively by subjecting the resultant cyclic dimer to ring-opening melt polymerization in the presence of a catalyst (for example, Sn-base catalyst). In the process, however, reaction procedures are complex and the polymer obtained becomes expensive. Additionally, the preparation process is melt polymerization and the polymerization product is pelletized as such. As a result, the product has a disadvantage that the catalyst used for the polymerization remains intact in the polyhydroxycarboxylic acid. Particularly, contamination of the catalyst into the product leads to a significant adverse effect depending upon the intended purpose of the polymer. For example, in the case of administering a slow-release drug to organisms, polyhydroxycarboxylic acid decomposes while the catalyst remains intact in the tissue of the organisms. Consequently, polyhydroxycarboxylic acid cannot be applied to such uses when the catalyst is toxic. Since the decomposition speed of polyhydroxycarboxylic acid differs depending upon the residual amount of the catalyst, the slow-release function of the drug cannot be fully exhibited. It has also been known that a large amount of the catalyst remaining in the polyhydroxycarboxylic acid leads to deterioration of properties in heat resistance test and weatherability test. Thus, impurities such as a catalyst which are contained in polyhydroxycarboxylic acid result in great restriction to the uses of the polymer.
On the other hand, it has been known a process for preparing polyhydroxycarboxylic acid in the absence of a catalyst such as the Sn-base catalyst. For example, a process for preparing a copolymer from glycolide and lactide in the presence of a strongly acidic ion exchange resin and a process for preparing a homopolymer or copolymer of lactic acid and glycolic acid by dehydration polycondensation in the absence of a catalyst, have been known. In any processes, however, the polymer obtained has a low molecular weight of 5,000-30,000, is unsatisfactory in light of its mechanical, physical and chemical properties, and thus is greatly restricted in uses.
U.S. Pat. No. 5,310,865 has disclosed a polymerization process for directly obtaining polyhydroxycarboxylic acid from hydroxycarboxylic acid or its oligomer by using a metal or metal compound of Sn, Ti or Ni as a catalyst, while removing formed water from the reaction system.
Polyhydroxycarboxylic acid having a sufficiently high molecular weight can also be obtained by the direct dehydration polymerization process. However, the catalyst used in the process remains in the product unless it is removed by some procedures and leads to the above problems.
In any case of preparing polyhydroxycarboxylic acid having a sufficiently high molecular weight, catalyst contamination is inevitable as mentioned above in the present state of the art unless successive purification is carried out.
Several processes have been known on the purification process for removing the catalyst in polyhydroxycarboxylic acid. For example, U.S. Pat. No. 4,960,866 has disclosed a process for once dissolving catalyst containing polyhydroxycarboxylic acid in a water-immiscible organic solvent, successively bringing the resulting solution into contact with an aqueous layer or water which contains inorganic acid, water-soluble organic acid or a water soluble complexing agent, separating the organic layer and isolating polyhydroxycarboxylic acid by a known process. U.S. Pat. No. 4,810,775 has described a purification process for dissolving catalyst containing polyhydroxycarboxylic acid in a good solvent and successively adding a precipitant in a turbulent shear field.
These processes can surely remove the catalyst from polyhydroxycarboxylic acid. However, these processes have the following problems in industry.
The process disclosed in U.S. Pat. No. 4,960,866 can be applied to any kind of polyhydroxycarboxylic acid. However, the catalyst removing efficiency of the process is not so good and thus the polyhydroxycarboxylic acid must be maintained at a low concentration in the solvent in order to obtain satisfactory efficiency for removing the catalyst, which leads to a problem of extremely poor volume efficiency. U.S. Pat. No. 4,960,866 has also described that halogenated hydrocarbon, for example, methylene chloride or chloroform can be preferably used as a solvent and that the concentration of polyhydroxycarboxylic acid in the solvent must be 10% or less. In the example, polyhydroxycarboxylic acid is treated by dissolving at a concentration of 1% in chloroform in order to satisfactorily remove the catalyst.
The process disclosed in U.S. Pat. No. 4,810,775 carries out precipitation and purification of polyhydroxycarboxylic acid at the same time and is simple as compared with the process of U.S. Pat. No. 4,960,866. However, the process of U.S. Pat. No. 4,810,775 has a problem of requiring specific facilities.
As mentioned above, any process has both merits and demerits, and a satisfactory purification process of polyhydroxycarboxylic acid has not yet been found.